The invention relates to a structure producing lift in the flow of a fluid medium and whose tip vortex which it produces in this flow is reduced.
It may find an application in numerous fields, such as aeroplane wings and ailerons, some sea-going vehicle appendages such as fins, anti-roll bars, rudders, propeller blades, etc. and some deflectors used for acoustical tests at sea.
Tip vortex is produced at the free end of all these bearing structures and results from a movement of the fluid transverse to the direction of flow, from the high-pressure side to the low-pressure side moving round the free edge of the structure. Tip vortex generally has a small cross-section but a high rotation speed. It is very coherent, i.e. its energy is concentrated in a reduced volume; it is not particularly subject to dissipation arising from viscosity and subsists for quite a time. It is therefore found downstream of the flow far from the structure which created it, which is prejudicial to acoustic discretion if sought or if acoustic measurements are to be taken at this point; and as the speed of the vortex is even greater the nearer one gets to the centre, the low-pressure formed may reach the point of cavitation of the liquid, which intensifies the noise still further and may well erode the structure itself.
Tip vortex is found even when a transverse plate is installed at the end of the bearing structure so as to improve lift or to reduce drag by reducing the transverse flow of fluid between the high-pressure side and the low-pressure side; it then forms at the edge of the plate and moreover has significantly greater coherence than on a streamlined end of a structure such as a traditional wing.
Measures intended to reduce tip vortex have been conceived: thus the surface portion on which the vortex formed has been roughened, in the hope of increasing friction at this point and of extending the viscous core which forms in the centre of the vortex so as to increase dissipation thereby; another idea has been to carry out drilling at the tip of the structure to produce localised pressure losses; but the effectiveness of these measures remains unproven, with the result that they are not generally used.
The abstract for patent JP 04 314 693 A (published in Patent Abstracts of Japan, vol.017, no. 136 (M-1384) on Mar. 19, 1993) describes a propeller blade the end edge of which is toothed in ridges which diffuse tip vortex.
The purpose of the object of the invention is to reduce the effects of tip vortex by a more effective attack on its coherence.
The device used is placed on a free end contour of a structure for producing lift in a fluid medium subject to a flow, a structure having a main elongation direction at the end of which this contour is located; the device consists of a plurality of ridges arranged in succession in the direction of the flow and oblique to the direction of the flow, or possibly perpendicular to this direction. The ridges are installed on a plate added to the free end of the bearing structure to increase lift and approximately perpendicular to a main portion of the structure. The plate bears the free end contour, and the ridges are located on an edge of the plate extending approximately in the direction of flow and located on a low-pressure side of the fluid delimited by the main portion of the structure.
The ridges have the effect of dividing the vortex into several smaller vortices which are juxtaposed, such that they counter each other and weaken each other; moreover, turbulences will be produced at the points where the vortices appear and will have the effect of reducing their intensity.
It may therefore be assumed that risks of cavitation will be smaller and that vortex remanence downstream of the bearing structure will be reduced.